ROT and meldew in the attic and possibly the associated walls. I rebuilt a bathroom twice and kept getting mildew and even mushrooms in some places. I was sure I must of missed properly soldering a pipe joint creating a leak, but could never find one.Then I discovered that the attic ceiling fan had been disturbed and its connecting exhaust line was not connected anymore to its fan. I reconnected the exhaust line and never had a mildew problem again. Now the issue I am referring to is a bathroom fan that vents through the attic, not an attic fan itself. Temperature and moisture control in your attic is very important and MUST be properly maintained. Too much ventilation in the attic is just as bad as too little. Signed DDA a HVACR guy.